Differential amplifiers are used in high quality microphone amplifiers, many of which are required to meet the requirements of professional broadcasters. To provide common mode rejection, it is known to use microphone amplifiers that include transformers for coupling the microphone output to the amplifying electronics. However the frequency response of transformers is not particularly good therefore there has been a tendency to move away from this solution and to make use of differential balanced amplifiers.
A known approach includes the use of two amplifier paths followed by a precision subtracter. The idea of this approach is that any signal that is common to the amplifier paths gets removed in the subtracter. However, this presents a problem in that the amplifier does pass the common mode signal therefore the voltage to be developed becomes that of the differential voltage plus the common mode voltage. Thus, if such an amplifier is to provide sufficient gain to be useful, it is likely that the peak voltage required due to the presence of the common mode signal will be relatively large compared to that provided by conventional supply rails.
An alternative approach is to provide a limiter so that it is possible to turn down the gain at the front end when large signals are present. However, such an approach impacts upon the dynamic range in that the original dynamic range will require a reserve thereby adversely affecting the overall performance. Furthermore, it is not practicable to make electronic variable resistances accurate enough to be used for controlling the known front end input.